Abstract Apolipoprotein C-III (apoC-III) is increasingly recognized as an important determinant of dyslipidemia and cardiovascular risk. Although apoC-III is typically measured as total plasma apoC-III concentration, it is present in blood in several post-translational proteoforms - with the most abundant forms containing zero, one or two sialic acid molecules. However, the relationships between apoC-III proteoforms and patterns of dyslipidemia, atherosclerosis and cardiovascular disease (CVD) risk are largely unknown. We found that higher relative amounts of apoC-III with two sialic acids (but not 0 or 1) were linked in cross-sectional and prospective studies with lower triglyceride levels, and were associated with reduced risk for CVD events. Our data also indicate ratios of these proteoforms may vary with race and lipid-lowering therapies and that proteoforms of both apoC-I and C-II also demonstrate unique relationships with blood lipid concentrations. The Multi-Ethnic Study of Atherosclerosis (MESA) is a large community based cohort study of CVD risk factors, subclinical measures of atherosclerosis and determinants of major CVD events. Importantly, the diverse ethnic and racial population within the study will permit a definitive study of these proteoforms and their relationships with lipid abnormalities, atherosclerosis and CVD events while exploring ethnic/racial differences. The present proposal will utilize a novel mass spectrometry immunoassay (MSIA) to investigate the relationships of apoC-III (and secondarily apo-CI and C-II) proteoforms with plasma lipid concentrations, progression of vascular calcification and carotid atherosclerosis, and development of major adverse cardiovascular events (MACE) within the MESA cohort. Total plasma apoC-III concentrations and apoC-III relative proteoform amounts will be determined by MSIA in baseline and 10-year follow-up plasma samples. Aim 1 will examine the cross-sectional and longitudinal associations between apoC-III measures and plasma lipids. Aim 2 will determine whether apoC-III measures predict baseline atherosclerosis and atherosclerosis progression evaluated from CT scans of coronary arteries and ultrasonography-derived carotid-intima-media thickness measured during MESA. Aim 3 will determine the relationship between apoC-III measures and incident MACE and total CVD events. This will be the first large study evaluating relationships between apoC proteoforms and CVD risk. As therapies are now being developed to specifically target apoC proteins to reduce triglycerides and CVD risk, a more definitive understanding of the complexity and clinical implications of the apoC family of proteins is needed. As diabetes and lipid-lowering therapies appear to have differential effects on apoC proteoforms, targeting therapy to specific proteoforms and/or subsets of individuals may be beneficial. As we have demonstrated that there are proteoforms of many other plasma proteins, this study may also help highlight the broader value of identifying variations in post-translational protein structure and their functional differences.